44
1 Section 3 Section 3 Section 3 Section 3 Section 3 MICROCONTROLLER INTERFACING CIRCUITS revolution Revolution Education Ltd. Business Innovation Centre, Innova Park, Mollison Avenue, Enfield, Middlesex, EN3 7XU Tel: 020 8350 1315 Fax: 020 8350 1351 Email: [email protected] Web: www.rev-ed.co.uk © copyright 2000 MICROCONTROLLER INTERFACING CIRCUITS What is a PIC Microcontroller? What is a PIC Microcontroller? What is a PIC Microcontroller? What is a PIC Microcontroller? What is a PIC Microcontroller? A PIC microcontroller is a single integrated circuit small enough to fit in the palm of a hand. ‘Traditional’ microprocessor circuits contain four or five separate integrated circuits - the microprocessor (CPU) itself, an EPROM program memory chip, some RAM memory and an input/output interface. With PIC microcontrollers all these functions are included within one single package, making them cost effective and easy to use. PIC microcontrollers can be used as the ‘brain’ to control a large variety of products. In order to control devices, it is necessary to interface (or ‘connect’) them to the PIC microcontroller. This section will help to enable those with limited electronics experience to successfully complete these interfacing tasks. Interfacing to the PIC Microcontroller Interfacing to the PIC Microcontroller Interfacing to the PIC Microcontroller Interfacing to the PIC Microcontroller Interfacing to the PIC Microcontroller This section explains how to interface many different input and output devices to the PIC microcontroller. BASIC code examples are provided for users of the Basic Stamp or PICAXE systems. Explanations of BASIC commands are provided in the Commands section (available separately). The interfacing circuits can also be used with any PIC microcontrollers such as the PIC16F84, although these microcontrollers may require programming in assembler code. This section is split into four subsections: Introduction to ‘standard’ interfacing circuits Output Device Interfacing Input Device Interfacing Advanced Component Interfacing

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  • 11111 Section 3Section 3Section 3Section 3Section 3

    MICROCONTROLLER INTERFACING CIRCUITS

    revolution Revolution Education Ltd. Business Innovation Centre, Innova Park, Mollison Avenue, Enfield, Middlesex, EN3 7XUTel: 020 8350 1315 Fax: 020 8350 1351 Email: [email protected] Web: www.rev-ed.co.uk

    © copyright 2000

    MICROCONTROLLERINTERFACING CIRCUITS

    What is a PIC Microcontroller?What is a PIC Microcontroller?What is a PIC Microcontroller?What is a PIC Microcontroller?What is a PIC Microcontroller?

    A PIC microcontroller is a single integrated circuit small enough to fit in the palm of a

    hand. ‘Traditional’ microprocessor circuits contain four or five separate integrated

    circuits - the microprocessor (CPU) itself, an EPROM program memory chip, some

    RAM memory and an input/output interface. With PIC microcontrollers all these

    functions are included within one single package, making them cost effective and easy

    to use.

    PIC microcontrollers can be used as the ‘brain’ to control a large variety of products. In

    order to control devices, it is necessary to interface (or ‘connect’) them to the PIC

    microcontroller. This section will help to enable those with limited electronics

    experience to successfully complete these interfacing tasks.

    Interfacing to the PIC MicrocontrollerInterfacing to the PIC MicrocontrollerInterfacing to the PIC MicrocontrollerInterfacing to the PIC MicrocontrollerInterfacing to the PIC Microcontroller

    This section explains how to interface many different input and output devices to the

    PIC microcontroller. BASIC code examples are provided for users of the Basic Stamp or

    PICAXE systems. Explanations of BASIC commands are provided in the Commands

    section (available separately). The interfacing circuits can also be used with any PIC

    microcontrollers such as the PIC16F84, although these microcontrollers may require

    programming in assembler code.

    This section is split into four subsections:

    • Introduction to ‘standard’ interfacing circuits

    • Output Device Interfacing

    • Input Device Interfacing

    • Advanced Component Interfacing

  • 22222 Section 3Section 3Section 3Section 3Section 3

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    revolution Revolution Education Ltd. Business Innovation Centre, Innova Park, Mollison Avenue, Enfield, Middlesex, EN3 7XUTel: 020 8350 1315 Fax: 020 8350 1351 Email: [email protected] Web: www.rev-ed.co.uk

    © copyright 2000

    Note on the BASIC Code SamplesNote on the BASIC Code SamplesNote on the BASIC Code SamplesNote on the BASIC Code SamplesNote on the BASIC Code Samples

    Simple BASIC code examples are provided within each subsection. The samples are

    not ‘complete’ programs but sections of code that can be included within a main

    program when using that particular component. When using these code samples it

    must be remembered that:

    1. Each pin should be set up as an input or output before using the code (stamp

    users only).

    2. If the hardware pins are changed from those given in the circuit diagrams it will

    be necessary to modify the pin numbers in the code.

    3. Any let dirs = or let pins = commands will adjust all 8 pins, in the port.

    4. Try to keep variables independant of each other. If a sub-procedure uses a

    variable, do not use the same variable anywhere else in the code. If the same

    variable must be used again, make sure there is no way it can clash with any

    other part of the code. This is the most common way of adding ‘hard-to-find’

    bugs into software code.

    Note on Component SelectionNote on Component SelectionNote on Component SelectionNote on Component SelectionNote on Component Selection

    For convenience and ease of understanding, a single device has been adopted when

    using standard interfacing components such as transistors and MOSFETS. For instance,

    the ‘standard’ transistor selected is the darlington device BCX38B. This does not mean

    that this device is the only transistor that can be used in all the transistor circuits, as it

    is not, but it is chosen because it is suitable for the majority of project work

    applications. All components listed are common devices that can be purchased from

    almost all electronics distributors.

  • 33333 Section 3Section 3Section 3Section 3Section 3

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    revolution Revolution Education Ltd. Business Innovation Centre, Innova Park, Mollison Avenue, Enfield, Middlesex, EN3 7XUTel: 020 8350 1315 Fax: 020 8350 1351 Email: [email protected] Web: www.rev-ed.co.uk

    © copyright 2000

    Standard Interfacing CircuitsStandard Interfacing CircuitsStandard Interfacing CircuitsStandard Interfacing CircuitsStandard Interfacing Circuits

    1. The Standard Transistor Interfacing Circuit

    2. Using a Darlington Driver IC

    3. The Standard Relay Interfacing Circuit

    4. The Standard FET Interfacing Circuit

    Output Device InterfacingOutput Device InterfacingOutput Device InterfacingOutput Device InterfacingOutput Device Interfacing

    1. LED

    2. Signal Lamp

    3. Buzzer

    4. Piezo Sounder & Speakers

    5. Solar & DC (“toy”) Motors

    6. Unipolar Stepper Motor

    7. Bipolar Stepper Motor

    8. Radio-Control Servo

    9. Counter Module

    10. Seven Segment Display

    11. Solenoid & Isonic Solenoid Valve

    12. Smart Wire / Springs

    Input Device InterfacingInput Device InterfacingInput Device InterfacingInput Device InterfacingInput Device Interfacing

    1. Switches

    2. Potentiometers

    3. Light Dependant Resistor (LDR)

    4. Thermistor

    Advanced Component InterfacingAdvanced Component InterfacingAdvanced Component InterfacingAdvanced Component InterfacingAdvanced Component Interfacing

    1. Liquid Crystal Display (LCD)

    2. Serial Communication with a Computer

  • 44444 Section 3Section 3Section 3Section 3Section 3

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    revolution Revolution Education Ltd. Business Innovation Centre, Innova Park, Mollison Avenue, Enfield, Middlesex, EN3 7XUTel: 020 8350 1315 Fax: 020 8350 1351 Email: [email protected] Web: www.rev-ed.co.uk

    © copyright 2000

    STANDARD INTERFACING CIRCUITS

    Standard Circuits 1 - The Transistor Interfacing CircuitStandard Circuits 1 - The Transistor Interfacing CircuitStandard Circuits 1 - The Transistor Interfacing CircuitStandard Circuits 1 - The Transistor Interfacing CircuitStandard Circuits 1 - The Transistor Interfacing Circuit

    Many output devices will require a transistor switching circuit. In most cases a

    darlington pair formed from two transistors is ideal.

    However this circuit requires that two separate transistors are used. It is possible to

    buy a device that contains the two transistors in a single package. This transistor is

    called the BCX38B, and can switch currents up to 800mA. This is the transistor used in

    all the circuits through this book.

    Note that it is usual to connect a back emf suppression diode across the output device.

    This is essential with devices such as relays, solenoids and motors which create a back

    emf when power is switched off. The diode type 1N4001 is the device recommended.

    0V

    Pin10k

    Outputdevice

    BC639

    Back EMFsuppressiondiode

    V+

    BC548B

    0V

    Pin10k

    Outputdevice

    BCX38B

    V+

  • 55555 Section 3Section 3Section 3Section 3Section 3

    MICROCONTROLLER INTERFACING CIRCUITS

    revolution Revolution Education Ltd. Business Innovation Centre, Innova Park, Mollison Avenue, Enfield, Middlesex, EN3 7XUTel: 020 8350 1315 Fax: 020 8350 1351 Email: [email protected] Web: www.rev-ed.co.uk

    © copyright 2000

    0V

    V+

    0V

    M

    M

    Pin 1

    Pin 2 ULN

    2003

    In 1

    In 2

    In 3

    In 4

    In 5

    In 6

    In 7

    Gnd

    Out 1

    Out 2

    Out 3

    Out 4

    Out 5

    Out 6

    Out 7

    V+

    1

    8

    16

    9

    0V

    V+

    0V

    M

    M

    Pin 1

    Pin 2 ULN

    2803

    In 1

    In 2

    In 3

    In 4

    In 5

    In 6

    In 7

    In 8

    Gnd

    Out 1

    Out 2

    Out 3

    Out 4

    Out 5

    Out 6

    Out 7

    Out 8

    V+

    1

    9

    18

    10

    Standard Circuits 2 - Using a Darlington Driver ICStandard Circuits 2 - Using a Darlington Driver ICStandard Circuits 2 - Using a Darlington Driver ICStandard Circuits 2 - Using a Darlington Driver ICStandard Circuits 2 - Using a Darlington Driver IC

    If a number of output devices are being controlled it may be necessary to use a

    number of output transistors. In this case it will often be more convenient to use a

    ULN2003 Darlington driver IC. This is simply a 16 pin ‘chip’ that contains 7

    darlington transistors similar in value to the BCX38B. The ‘chip’ also contains internal

    back emf suppression diodes and so no external 1N4001 diodes are required.

    A device called the ULN2803 Darlington Driver IC is also available. This is identical to

    the ULN2003 except that it is an 18 pin device and contains 8 darlington pairs instead

    of 7. If it is necessary to pass relatively high currents through a device it can be useful

    to ‘pair up’ drivers as shown with this circuit.

    A ULN2803 darlington driver is supplied prefitted to the PICAXE interface board.

  • 66666 Section 3Section 3Section 3Section 3Section 3

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    revolution Revolution Education Ltd. Business Innovation Centre, Innova Park, Mollison Avenue, Enfield, Middlesex, EN3 7XUTel: 020 8350 1315 Fax: 020 8350 1351 Email: [email protected] Web: www.rev-ed.co.uk

    © copyright 2000

    Standard Circuits 3 - The Relay Interfacing CircuitStandard Circuits 3 - The Relay Interfacing CircuitStandard Circuits 3 - The Relay Interfacing CircuitStandard Circuits 3 - The Relay Interfacing CircuitStandard Circuits 3 - The Relay Interfacing Circuit

    A relay can be used to switch higher power devices such as motors and solenoids. If

    desired, the relay can be powered by a separate power supply, so, for instance, 12V

    solenoids can be controlled by the microcontroller. Note the use of a back emf

    suppression diode across the relay contacts. This is to prevent damage to the transistor

    when the relay switches off. Diode type 1N4001 is suitable for this diode.

    0V

    Pin10k

    BCX38B

    1N4001 RL1

    5V

    Standard Circuits 4 - The Power MOSFET Interfacing CircuitStandard Circuits 4 - The Power MOSFET Interfacing CircuitStandard Circuits 4 - The Power MOSFET Interfacing CircuitStandard Circuits 4 - The Power MOSFET Interfacing CircuitStandard Circuits 4 - The Power MOSFET Interfacing CircuitPower MOSFETs can be used instead of darlington transistor pairs to switch medium

    power devices. The standard MOSFET circuit is shown below. The device IRF530 is a

    suitable power MOSFET to use in this circuit.

    Note that it is usual to connect a back emf suppression diode across the output device.

    This is essential with devices such as relays, solenoids and motors which create a back

    emf when power is switched off. The diode type 1N4001 is the device recommended.

    0V

    Pin

    +6V

    IRF530

    M1N4001

  • 77777 Section 3Section 3Section 3Section 3Section 3

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    © copyright 2000

    OUTPUT DEVICE INTERFACING

    Output Device 1 - Light Emitting Diode (LEDs)Output Device 1 - Light Emitting Diode (LEDs)Output Device 1 - Light Emitting Diode (LEDs)Output Device 1 - Light Emitting Diode (LEDs)Output Device 1 - Light Emitting Diode (LEDs)

    The PIC Microcontroller can sink (“absorb”) or source

    (“give out”) a small amount of current, which means that

    an LED can be connected directly to the output pin. A

    series resistor (value 330R) is also required to limit the

    current.

    LED connected to Ground Rail.

    To switch on LED - high 1

    To switch off LED - low 1

    LED connected to Power Rail.

    To switch on LED - low 1

    To switch off LED - high 1

    Bi-colour LEDs often contain both green and red LEDs connected in ‘inverse parallel’.

    This means if current flows one way through the device the LED lights green, and if

    current flows the other way the LED lights red. Therefore by using the sink/source

    capabilities of the PIC Microcontroller it is possible to light the LED in both colours.

    To switch on LED in red - high 0

    low 1

    To switch on LED in green - low 0

    high 1

    To switch off LED - low 0

    low 1

    or, high 0

    high 1

    0V

    Pin 1

    330R

    Pin 1

    5V

    330R

    Pin 0

    330R

    Red Green

    Bi-colour LED

    Pin 1

  • 88888 Section 3Section 3Section 3Section 3Section 3

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    © copyright 2000

    Output Device 2 - Signal LampOutput Device 2 - Signal LampOutput Device 2 - Signal LampOutput Device 2 - Signal LampOutput Device 2 - Signal Lamp

    To interface a signal lamp the standard transistor interfacing circuit is used. Note that if

    a different power supply is used for the signal lamp, the 0V rails of each power supply

    must be connected to provide a common reference.

    If a battery is used as the power supply, it is worth remembering that LEDs draw much

    less current than lamps. Therefore, if a simple ‘indicator’ is required, a LED will be a

    better solution than a lamp as the batteries will last far longer.

    To switch on Lamp - high 1

    To switch off Lamp - low 1

    0V

    Pin 110k

    Signallamp

    6V

    BCX38B

    signal lamp

    buzzer

    Output Device 3 - BuzzerOutput Device 3 - BuzzerOutput Device 3 - BuzzerOutput Device 3 - BuzzerOutput Device 3 - Buzzer

    To interface a buzzer the standard transistor interfacing circuit is used. Note that if a

    different power supply is used for the buzzer, the 0V rails of each power supply must

    be connected to provide a common reference.

    If a battery is used as the power supply, it is worth remembering that piezo sounders

    draw much less current than buzzers. Buzzers also just have one ‘tone’, whereas a

    piezo sounder is able to create sounds of many different tones.

    To switch on buzzer -high 1

    To switch off buzzer -low 1

    0V

    Pin10k

    Buzzer

    BCX38B

    6V

  • 99999 Section 3Section 3Section 3Section 3Section 3

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    © copyright 2000

    Output Devices 4 - Piezo Sounder & SpeakerOutput Devices 4 - Piezo Sounder & SpeakerOutput Devices 4 - Piezo Sounder & SpeakerOutput Devices 4 - Piezo Sounder & SpeakerOutput Devices 4 - Piezo Sounder & Speaker

    A piezo sounder or speaker can be used to produce many different sounds, whereas a

    buzzer can only produce a single tone. Buzzers produce a noise when power is

    applied, but a piezo or speaker requires a pulsed signal to generate the noise.

    Fortunately this is very easy to generate from the microcontroller by using the BASIC

    ‘sound’ command.

    To produce a note of pitch 100, length 50 on pin 1 -

    sound 1, (100,50)

    To produce a varying noise using variable b1 -

    for b1 = 1 to 100

    sound 1, (b1,25)

    next b1

    Pin 1

    0V

    Pin 1

    0V

    +

    40R

    10uF

  • 1010101010 Section 3Section 3Section 3Section 3Section 3

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    © copyright 2000

    Output Devices 5 - Solar & DC “Toy” MotorsOutput Devices 5 - Solar & DC “Toy” MotorsOutput Devices 5 - Solar & DC “Toy” MotorsOutput Devices 5 - Solar & DC “Toy” MotorsOutput Devices 5 - Solar & DC “Toy” Motors

    Many projects require the use of a cheap dc motor to create rotational movement.

    There are a number of ways motors can be interfaced to the microcontroller.

    This circuit uses a darlington transistor to switch the motor on and off. This circuit will

    work with ‘solar’ motors, but may not function correctly with cheap dc ‘toy’ motors.

    This is because this type of motor introduces a lot of electrical ‘noise’ on to the power

    rails. This noise can affect the microcontroller, and in some cases can completely stop

    the control program functioning.

    0V

    Pin 110k

    Solarmotor

    6V

    0V

    BCX38B

    M1N4001

    solar motor

    Electrical noise can be reduced by

    soldering suppression capacitors

    across the motor contacts, as

    shown. Use a 220nF polyester

    (non polarised) capacitor.

    In order to switch medium power motors, a power MOSFET is used instead of a

    darlington transistor. The MOSFET circuit is shown below. The device IRF530 is a

    suitable power MOSFET to use in this circuit.

    0V

    Pin

    +6V

    IRF530

    M1N4001

  • 1111111111 Section 3Section 3Section 3Section 3Section 3

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    © copyright 2000

    On many occasions it may be necessary to control two motors. A convenient and

    cheap approach would be to use a motor driver IC such as the L293D. This IC will

    allow control of two dc motors, using four data lines from the microcontroller.

    Naturally, if only one motor is to be controlled then only two output lines are used.

    Both inputs low - motor halt

    First output high, second output low - motor forward

    First output low, second output high - motor reverse

    Both inputs high - motor halt

    Changing the states of the input pins has the effect of altering the direction of current

    flow through the motor, as shown below.

    0V 0V

    M

    Pin 4

    L293D

    5V

    In 1

    Out 1

    0V

    0V

    Out 2

    In 2

    V+

    5V

    In 3

    Out 3

    0V

    0V

    Out 4

    In 4

    5V

    1

    8

    16

    9Pin 5

    To V2+

    V2+

    Motor A M

    Pin 6

    Pin 7

    Motor B

    Note that the L293D will become warm with continuous use. A heatsink bonded onto

    the top of the chip will help keep it cool.

    Current flow

  • 1212121212 Section 3Section 3Section 3Section 3Section 3

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    © copyright 2000

    0V

    Pin 010k

    BCX38B

    1N4001 RL1

    M

    5VV+

    Pin 110k

    RL2

    1/2

    2/11/1

    GNDContacts 2/2 not used

    relay0V

    Pin 110k

    BCX38B

    1N4001 RL1

    M

    6V

    6Vbattery

    One way to prevent electrical noise affecting the microcontroller is to use separate

    power supplies for the ‘control’ electronics and the motor. For example, a PP3 battery

    may be chosen to power the microcontroller and 4xAA cells to power the motors.

    Naturally it will be necessary to ‘link’ the two circuits so that the motor can be

    controlled. A relay is an ideal component to do this.

    The above circuit will only switch the motor on and off. If the motor is required to run

    in both directions (forwards and reverse), two relays can be used as shown.

  • 1313131313 Section 3Section 3Section 3Section 3Section 3

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    © copyright 2000

    Output Device 6 - Unipolar stepper motorOutput Device 6 - Unipolar stepper motorOutput Device 6 - Unipolar stepper motorOutput Device 6 - Unipolar stepper motorOutput Device 6 - Unipolar stepper motor

    Stepper motors are very accurate motors that are commonly used in computer disk

    drives, printers and clocks. Unlike dc motors, which spin round freely when power is

    applied, stepper motors require that their power supply be continuously pulsed in

    specific patterns. For each pulse, the stepper motor moves around one ‘step’, often 7.5

    degrees (giving 48 steps in a full revolution).

    There are two main types of stepper motors - Unipolar and Bipolar. Unipolar motors

    usually have four coils which are switched on and off in a particular sequence. Bipolar

    motors have two coils in which the current flow is reversed in a similar sequence. Use

    of bipolar motors is covered in the next section.

    Each of the four coils in a unipolar stepper motor must be switched on and off in a

    certain order to make the motor turn. Many microprocessor systems use four output

    lines to control the stepper motor, each output line controlling the power to one of the

    coils.

    As the stepper motor operates at 12V, the standard transistor circuit is required to

    switch each coil. As the coils create a back emf when switched off, a suppression diode

    on each coil is also required. The table below show the four different steps required to

    make the motor turn.

    Step Coil 1 Coil 2 Coil 3 Coil 4

    1 1 0 1 0

    2 1 0 0 1

    3 0 1 0 1

    4 0 1 1 0

    1 1 0 1 0

    Look carefully at the table, and notice that a pattern is visible. Coil 2 is always the

    opposite (or logical NOT) of coil 1. The same applies for coils 3 and 4. It is therefore

    possible to cut down the number of microcontroller pins required to just two by the

    use of two additional NOT gates.

    stepper motor

    +12V

  • 1414141414 Section 3Section 3Section 3Section 3Section 3

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    © copyright 2000

    Fortunately the darlington driver IC ULN2003 can be used to provide both the NOT

    and darlington driver circuits. It also contains the back emf suppression diodes so no

    external diodes are required. The complete circuit is shown below.

    Before programming, there is another pattern to notice in the stepping sequence. Look

    at this table, which just shows coil 1 and coil 3.

    Step Coil 1 Coil 3 Change

    1 1 1

    coil 3

    2 1 0

    coil 1

    3 0 0

    coil 3

    4 0 1

    coil 1

    1 1 1

    Notice the change from step 1 to step 2, just coil 3 changes. Then look at the next

    change - just coil 1 changes. In fact the two coils take it ‘in turns’ to change from high

    to low and back again. This high-low-high changing can be described as ‘toggling’

    state. This makes the programming very simple by using the BASIC toggle command.

    steps: toggle 1 ‘ Toggle pin 1

    pause 200 ‘ Wait 200 ms

    toggle 2 ‘ Toggle pin 2

    pause 200 ‘ Wait 200ms

    goto steps ‘ Loop

    Note: If stepper motor ‘wobbles’, try adjusting wire polarity.

    0V

    To 11

    Pin 2 ULN

    2003

    In 1

    In 2

    In 3

    In 4

    In 5

    In 6

    In 7

    Gnd

    Out 1

    Out 2

    Out 3

    Out 4

    Out 5

    Out 6

    Out 7

    Diode

    1

    8

    16

    9

    To 10NC

    Pin 1

    1k

    1k

    +12V

    NC

    To 1

    To 4

    1k

    1k+5V

    BRN

    BLK

    ORG

    YEL

    +12V(power supply)

    RED

    Stepper motor

    N.B. colours of steppermotor leads may vary

  • 1515151515 Section 3Section 3Section 3Section 3Section 3

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    © copyright 2000

    Output Device 7 - Bipolar Stepper motorOutput Device 7 - Bipolar Stepper motorOutput Device 7 - Bipolar Stepper motorOutput Device 7 - Bipolar Stepper motorOutput Device 7 - Bipolar Stepper motor

    Stepper motors are very accurate motors that are commonly used in computer disk

    drives, printers and clocks. Unlike dc motors, which spin round freely when power is

    applied, stepper motors require that their power supply be continuously pulsed in

    specific patterns. For each pulse, the stepper motor moves around one ‘step’, often 7.5

    degrees (giving 48 steps in a full revolution).

    There are two main types of stepper motors - Unipolar and Bipolar. Unipolar motors

    usually have four coils which are switched on and off in a particular sequence. Bipolar

    motors have two coils in which the current flow is reversed in a similar sequence. Use

    of unipolar motors is covered in the previous pages.

    The bipolar stepper motor has two coils that must be controlled so that the current

    flows in different directions through the coils in a certain order. The changing magnetic

    fields that these coils create cause the rotor of the motor to move around in steps.

    The circuit that is normally used to control one of the coils is shown below. Notice

    how there are four ‘control’ transistors, that are switched on in ‘pairs’. Therefore with

    two coils there are four control transistor pairs (Q1-Q4) which must be switched on

    and off in a certain sequence.

    motor coil

    Q1

    Q2

    Q2

    Q1

    12V

    0V

    Current flow

  • 1616161616 Section 3Section 3Section 3Section 3Section 3

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    © copyright 2000

    Notice that as the coils create a back emf when switched off 8 suppression diodes (4

    on each coil) are also required.

    The table below show the four different steps required to make the motor turn

    Step Q1 Q2 Q3 Q4

    1 1 0 1 0

    2 1 0 0 1

    3 0 1 0 1

    4 0 1 1 0

    1 1 0 1 0

    Fortunately the motor driver L293D has been specifically designed to provide this

    transistor switching circuit. The L293D contains all 8 transistors and diodes within one

    16 pin package.

    Four pins from the microcontroller are connected to the four transistor ‘pairs’ via IC

    pins 2, 7, 10 and 15.

    0V 0V

    M

    Pin 4

    L293D

    5V

    In 1

    Out 1

    0V

    0V

    Out 2

    In 2

    V+

    5V

    In 3

    Out 3

    0V

    0V

    Out 4

    In 4

    5V

    1

    8

    16

    9Pin 5

    To V2+

    V2+

    Motor A M

    Pin 6

    Pin 7

    Motor B

    This sample procedure makes the motor spin 100 steps to the left and then 100 steps

    to the right by using two sub-procedures. lstep causes the motor to move one step to

    the left, rstep causes the motor to move one step to the right. Variable b1 is used to

    store the step position and so should not be used elsewhere in the program.

    main: for b3 = 0 to 99 ‘ start a for...next loop

    gosub lstep ‘ call left step sub-procedure

    next b3 ‘ next loop

    for b3 = 0 to 99 ‘ start a for...next loop

    gosub rstep ‘ call left step sub-procedure

    next b3 ‘ next loop

    lstep: let b1 = b1 + 1 ‘ add 1 to variable b1

    goto step ‘ goto the lookup table

    rstep: let b1 = b1 - 1 ‘ subtract 1 from variable b1

    step: let b1 = b1 & 2 ‘ mask lower two bits of b1

    lookup b1,(%1010,%1001,%0101,%0110),b2 ‘ lookup code into b2

    let pins = b2 ‘ output b2 onto control lines

    return

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    © copyright 2000

    Output Device 8 - Radio Control ServoOutput Device 8 - Radio Control ServoOutput Device 8 - Radio Control ServoOutput Device 8 - Radio Control ServoOutput Device 8 - Radio Control Servo

    Servos are used in most radio controlled

    cars and planes to control the steering

    mechanism. They are accurate devices that

    always rotate the same amount for a given

    signal, and so are ideal for use in many

    automated machines.

    A typical servo has just three connection wires, normally red, black and white (or

    yellow). The red wire is the 5V supply, the black wire is the 0V supply, and the white

    (or yellow) wire is for the positioning signal.

    The positioning signal is a pulse between 0.75 and 2.25 milliseconds (ms) long,

    repeated about every 18ms (so there are roughly 50 pulses per second). With a 0.75ms

    pulse the servo moves to one end of its range, and with a 2.25ms pulse the servo

    moves to the other. Therefore, with a 1.5ms pulse, the servo will move to the central

    position. If the pulses are stopped the servo will move freely to any position.

    Unfortunately servos require a large current (up to 1A) and also introduce a large

    amount of noise on to the power rail. Therefore in most cases the servo should be

    powered from a separate power supply, as shown below. Remember that when using

    two power supplies the two 0V rails must be joined to provide a common reference

    point.

    loop: servo 4,75 ‘ move servo to one end

    pause 2000 ‘ wait 2 seconds

    servo 4,150 ‘ move servo to centre

    pause 2000 ‘ wait 2 seconds

    servo 4,225 ‘ move servo to other end

    pause 2000 ‘ wait 2 seconds

    goto loop ‘ loop back to start

    Pin330R

    W

    R

    B

    SERVO

    6V SUPPLY

    V2+

    6V 0V

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    Output Device 9 - Counter moduleOutput Device 9 - Counter moduleOutput Device 9 - Counter moduleOutput Device 9 - Counter moduleOutput Device 9 - Counter module

    The Counter Module is a numeric LCD display module that can be used to show a

    ‘counter’ value. To increment the counter a pulse (between 1 and 1.5V) must be

    applied to the counter pad 3. As the PIC microcontroller operates at 5V a potential

    divider formed from resistors must be used to reduce the PIC microcontroller output

    signal to 1.5V. As the counter uses it’s own, internal, 1.5V battery, the two 0V rails must

    also be connected.

    3k3

    Pin 1

    1k

    0V

    Counter

    1 3

    reset

    2

    0V count

    To increment counter: pulsout 1,100

    To reset the counter, a second potential divider is added and connected to pin 2.

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    Output Device 10 - Seven Segment DisplayOutput Device 10 - Seven Segment DisplayOutput Device 10 - Seven Segment DisplayOutput Device 10 - Seven Segment DisplayOutput Device 10 - Seven Segment Display

    Pin 2

    Pin 116

    15

    14

    13

    12

    11

    10

    9

    1

    2

    3

    4

    5

    6

    7

    8Pin 0

    +5V

    0V

    Pin 3

    fgabcde

    B

    C

    LT

    BK

    ST

    D

    A

    Gnd

    +5V

    f

    g

    a

    b

    c

    d

    e

    a

    g

    de

    f b

    c

    4511B

    This code example counts through the digits 0 to 9

    main: for b1 = 0 to 9 ‘ Set up a for...next loop using variable b1

    let pins=b1 ‘ Output b1 onto the four data lines

    pause 1000 ‘ Pause 1 second

    next b1 ‘ Next

    goto main ‘ Loop back to start

    A seven segment display contains seven LED

    ‘bars’ that can be lit up in different

    combinations to show the ten digits 0 to 9. In

    theory each ‘bar’ could be connected to one

    microcontroller output pin, but this would

    use up 7 of the 8 available pins!

    A better solution is to use a dedicated integrated circuit, such as the CMOS 4511B to

    control the seven segment display. This IC controls the seven segment display

    according to the binary ‘code’ on the four data lines. This system uses four pins rather

    than 7.

    IMPORTIMPORTIMPORTIMPORTIMPORTANT NANT NANT NANT NANT NOOOOOTETETETETE - Seven segment displays are available in two types, called ‘commoncathode’ and ‘common anode’. The following circuits will only work with a ‘commoncathode’ type display. Use the manufacturer’s datasheet to determine the pinoutarrangement of the LED bars.

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    Another possible solution is to use the CMOS 4026B to control the seven segment

    display. This system uses just two pins to control the display. The reset pin is used to

    reset the display to 0, the clock pin is then used to increment the digit up from 0. This

    means to display the digit ‘4’ it is necessary to reset and then pulse the clock line 4

    times. In reality this means that the display shows the digits 0-1-2-3-4, but, as they are

    clocked extremely rapidly, the human eye cannot see the changes, and so the number

    ‘4’ seems to appear immediately!

    This code example uses sub-procedure ‘clock’ to display the digit ‘4’, which is stored in

    the variable b1.

    ‘This is the sub-procedure

    clock: pulsout 0,10 ‘ reset display to 0

    if b1 = 0 goto endclk ‘ if b1 = 0 then return

    for b3 = 1 to b1 ‘ start a for...next loop

    pulsout 1,10 ‘ pulse clock line

    next b3 ‘ next loop

    endclk: return ‘ return from sub-procedure

    This is the main code

    main: let b1 = 4 ‘ give variable b1 the value 4

    gosub clock ‘ call sub-procedure

    pause 1000 ‘ wait 1 second

    goto main ‘ loop

    Pin 016

    15

    14

    13

    12

    11

    10

    9

    1

    2

    3

    4

    5

    6

    7

    8

    +5V

    0V

    cbead

    Clock

    Out

    f

    g

    Gnd

    +5V

    Reset

    c

    b

    e

    a

    d

    a

    g

    de

    f b

    c

    4026

    fg

    Pin 1

    To7 segmentdisplay

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    © copyright 2000

    This system can be expanded to two digits by adding a second 4026B IC and a second

    seven segment display, as shown in the diagram below. No changes to the code are

    required, just give the variable b1 a value between 0 and 99 and the number will be

    displayed on the two displays when sub-procedure ‘clock’ is called.

    Pin 016

    15

    14

    13

    12

    11

    10

    9

    1

    2

    3

    4

    5

    6

    7

    8

    +5V

    0V

    Clock

    Out

    f

    g

    Gnd

    +5V

    Reset

    c

    b

    e

    a

    d4026B

    16

    15

    14

    13

    12

    11

    10

    9

    1

    2

    3

    4

    5

    6

    7

    8

    Clock

    Out

    f

    g

    Gnd

    +5V

    Reset

    c

    b

    e

    a

    d

    4026B

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    Output Device 11 - Solenoid & Solenoid ValvesOutput Device 11 - Solenoid & Solenoid ValvesOutput Device 11 - Solenoid & Solenoid ValvesOutput Device 11 - Solenoid & Solenoid ValvesOutput Device 11 - Solenoid & Solenoid Valves

    A solenoid consists of a steel plunger inside an electric coil which is wrapped around a

    tube. When the coil is energised a magnetic field is created, and this draws the plunger

    into the tube. When the coil is de-energised a spring pushes the plunger back out of

    the tube.

    To control a solenoid the standard MOSFET circuit is used.

    The isonic solenoid valve can be used to control air flow through a pneumatic system.

    Isonic valves are ideal for battery operated products as operate at a low voltage and

    draw much less current than traditional solenoid valves. The standard transistor

    switching circuit can be used to drive the isonic valve.

    To switch the solenoid on - high 1

    To switch the solenoid off - low 1

    0V

    Pin 110k

    1N4001 Solenoidvalve

    5V

    solenoid

    0V

    Pin 1

    +6V

    IRF530

    1N4001

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    smartwire

    Output Device 12 -Output Device 12 -Output Device 12 -Output Device 12 -Output Device 12 -Smart Wire & Smart SpringsSmart Wire & Smart SpringsSmart Wire & Smart SpringsSmart Wire & Smart SpringsSmart Wire & Smart Springs

    Shape Memory Alloy wire or springs are ‘smart’ materials that can be used to create

    mechanical actuation (movement). When an electric current is passed through the wire

    it heats up and so contracts with a large pulling force. When the current is removed the

    wire cools and so expands again (a ‘traditional’ steel spring is sometimes used to pull

    the smart wire/spring taut as it cools).

    Smart wire or springs draw a relatively large current, and so the standard FET

    interfacing circuit should be used to interface to the microcontroller.

    To make the wire / spring contract - high 1

    To allow the wire / spring to expand again - low 1

    0V

    Pin 1

    +6V

    IRF530

    1N4001 smartwire

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    INPUT DEVICE INTERFACING

    Input Device 1 - SwitchesInput Device 1 - SwitchesInput Device 1 - SwitchesInput Device 1 - SwitchesInput Device 1 - Switches

    There are a large variety of switches available, but the majority all have two ‘contacts’

    which are either ‘open’ (off) or ‘closed’ (on). The two circuits shown below can be used

    with almost all switches.

    With this circuit the input pin is low when the switch is open and high when theswitch is closed.

    Goto ‘jump’ when switch is open: if pin0 = 0 then jump

    Goto ‘jump’ when switch is closed: if pin0 = 1 then jump

    5V

    0V

    10k

    1kPin 0

    5V

    0V

    10k

    1kPin 0

    With this circuit the input pin is high when the switch is open and low when theswitch is closed.

    Goto ‘jump’ when switch is open: if pin0 = 1 then jump

    Goto ‘jump’ when switch is closed: if pin0 = 0 then jump

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    © copyright 2000

    5V

    0V

    10k

    Pin 0

    330R

    Pin 1

    Switch BounceSwitch BounceSwitch BounceSwitch BounceSwitch Bounce

    All mechanical switches ‘bounce’ when the switch opens or closes. This means that the

    switch contacts ‘bounce’ against each other before settling. As the PIC microcontroller

    operates so quickly it is possible that in some programs the microcontroller may

    register 2 or 3 of these ‘bounces’ instead of just registering one ‘push’.

    The simplest way to debounce a circuit

    is to simply add a time delay (pause

    100) after the if... command. If the

    section of code after the push is quite

    long this time delay will occur

    naturally (as the other code

    commands are carried out) and so is

    unnecessary. However if the code does

    not have a long delay, as in the

    following example, a pause command

    can be used instead.

    The following two programs show the effect of switch bouncing. The program should

    light the LED on pin1 when the switch connected to pin0 has been pressed more than

    5 times. However, the first listing may not work correctly, because the microcontroller

    may count ‘bounces’ rather than actual pushes, and so the LED may light prematurely.

    init: let b0 = 0

    main: if pin 1 = 1 then add

    goto main

    add: let b0 = b0 + 1

    if b0 < 5 then main

    high 1

    goto main

    init: let b0 = 0

    main: if pin 1 = 1 then add

    goto main

    add: pause 100 ‘short delay here

    let b0 = b0 + 1

    if b0 < 5 then main

    high 1

    goto main

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    Input Device 2 - PotentiometerInput Device 2 - PotentiometerInput Device 2 - PotentiometerInput Device 2 - PotentiometerInput Device 2 - Potentiometer

    A potentiometer (or ‘variable resistor’)

    has a spindle that can be moved to change

    the resistance value of the potentiometer.

    This can be used to measure rotational or

    linear movement.

    The readADC command is used to measure the value of the resistance by carrying out

    an Analogue to Digital Conversion. The value of the resistance is given a ‘value’

    between 0 and 255 which is then stored in a variable. After storing the reading in the

    variable, the if...then command can be used to perform different functions.

    The program below lights three different LEDs (connected to pins 1, 2 and 3),

    depending on the analogue sensor reading.

    main: readadc 0,b1 ‘ read value on pin0 into variable b1

    if b1

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    ANAL

    OGUE

    VALU

    EPIC

    ANAL

    OGUE

    CALIB

    RATIO

    N BOA

    RD

    0VIN

    5V

    0V IN5V

    Input Device 3 - Light Dependant Resistor (LDR)Input Device 3 - Light Dependant Resistor (LDR)Input Device 3 - Light Dependant Resistor (LDR)Input Device 3 - Light Dependant Resistor (LDR)Input Device 3 - Light Dependant Resistor (LDR)

    A Light Dependant Resistor (LDR) is a resistor that changes in value according to the

    light falling on it. A commonly used device, the ORP-12, has a high resistance in the

    dark, and a low resistance in the light. Connecting the LDR to the microcontroller is

    very straight forward, but some software ‘calibrating’ is required.

    It should be remembered that the LDR response is not linear, and so the readings will

    not change in exactly the same way as with a potentiometer. In general there is a larger

    resistance change at brighter light levels. This can be compensated for in the software

    by using a smaller range at darker light levels. Experiment to find the most

    appropriate settings for the circuit.

    main: readadc 0,b1 ‘ read the value

    if b1

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    Input Device 4 - ThermistorInput Device 4 - ThermistorInput Device 4 - ThermistorInput Device 4 - ThermistorInput Device 4 - Thermistor

    A thermistor is a resistor that changes in value according to it’s heat. In actual fact all

    resistors change in value as they heat up or cool down, but thermistors are

    manufactured to show a large resistance change. Connecting the thermistor to the

    microcontroller is very straight forward, but some software ‘calibrating’ is required.

    It should be remembered that the thermistor response is not linear, and so the

    readings will not change in exactly the same way as with a potentiometer. In general

    there is a larger resistance change at lower temperatures. This can be compensated for

    in the software by using a smaller range at higher temperatures. Experiment to find the

    most appropriate settings for the circuit.

    main: readadc 0,b1 ‘ read the value

    if b1

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    ADVANCED COMPONENT INTERFACING

    Advanced Interfacing 1 - LCD DisplayAdvanced Interfacing 1 - LCD DisplayAdvanced Interfacing 1 - LCD DisplayAdvanced Interfacing 1 - LCD DisplayAdvanced Interfacing 1 - LCD Display

    A Liquid Crystal Display is an electronic device that can be

    used to show numbers or text. There are two main

    types of LCD display, numeric displays (used in

    watches, calculators etc) and alphanumeric

    text displays (often used in devices such as

    photocopiers and mobile telephones).

    The display is made up of a number of shaped ‘crystals’. In numeric displays these

    crystals are shaped into ‘bars’, and in alphanumeric displays the crystals are simply

    arranged into patterns of ‘dots’. Each crystal has an individual electrical connection so

    that each crystal can be controlled independently. When the crystal is ‘off’ (i.e. when

    no current is passed through the crystal) the crystal reflect the same amount of light as

    the background material, and so the crystals cannot be seen. However when the crystal

    has an electric current passed through it, it changes shape and so absorbs more light.

    This makes the crystal appear darker to the human eye - and so the shape of the dot or

    bar can be seen against the background.

    It is important to realise the difference between a LCD display and an LED display. An

    LED display (often used in clock radios) is made up of a number of LEDs which

    actually give off light (and so can be seen in the dark). An LCD display only reflects

    light, and so cannot be seen in the dark.

    LCD CharactersLCD CharactersLCD CharactersLCD CharactersLCD Characters

    The table on the next page shows the characters available from a typical LCD display.

    The character ‘code’ is obtained by adding the number at the top of the column with

    the number at the side of the row.

    Note that characters 32 to 127 are always the same for all LCDs, but characters 16 to 31

    & 128 to 255 can vary with different LCD manufacturers. Therefore some LCDs will

    display different characters from those shown in the table.

    Characters 0 to 15 are described as ‘user-defined’ characters and so must be defined

    before use, or they will contain ‘randomly shaped’ characters. For details on how to

    use these characters see the LCD manufacturers data sheets.

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    © copyright 2000

    0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240

    0

    1

    2

    3

    4

    5

    6

    7

    8

    9

    10

    11

    12

    13

    14

    15

    Row

    Val

    ue

    Column Value

    CGRAM(1)

    CGRAM(2)

    CGRAM(3)

    CGRAM(4)

    CGRAM(5)

    CGRAM(6)

    CGRAM(7)

    CGRAM(8)

    CGRAM(1)

    CGRAM(2)

    CGRAM(3)

    CGRAM(4)

    CGRAM(5)

    CGRAM(6)

    CGRAM(7)

    CGRAM(8)

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    Start with a piece of paper, on which one letter is written. Place the card over the paper,

    and the letter will be visible because it shows through the ‘display window’. Remove

    the card, write another letter, replace the card and they will both be visible. In fact all

    of the first sixteen letters will be visible, but the seventeenth will not, as the ‘display

    window’ is only wide enough for 16 letters.

    Blank ‘paper’

    First letter can be seen

    a

    Next letter can be seen

    a b

    17th letter cannot be seen as it is ‘outside’ the display window

    a b c d e f g h i j k l m n o p q r s t

    The operation of the display is quite complex as the display can actually store more

    characters than can be displayed at once. A simple model makes this easier to

    understand. Imagine a piece of paper with a row of letters written across it. If a piece of

    card is taken, which has a ‘window’ cut in it, and the card is placed over the paper, only

    some of the letters will be visible. The other letters are still there, it’s just that they

    cannot be seen. This is how a LCD display works - it stores a lot of characters, but only

    shows a few, through the ‘display window’, at once

    20 letters stored in display memory

    a b c d e f g h i j k l m n o p q r s t

    Only 16 letters can be seen at one time

    a b c d e f g h i j k l m n o p

    b c d e f g h i j k l m n o p q

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    To be able to see the seventeenth letter it is necessary to move (or ‘scroll’) the display

    window one place to the right, but this will also mean that the first letter can no

    longer be seen. Advantage can be taken of this ‘moving’ window method to make long

    messages appear to scroll across the LCD screen. To do this a long message is written

    into the LCD memory, and then the display window is repeatedly scrolled across the

    message. This is equivalent to ‘pulling’ the paper under the window to show the long

    message. The LCD window does not ‘physically’ move - so to anyone watching the

    LCD the letters ‘appear’ to be moving to the right.

    a b c d e f g h i j k l m n o p q r s t

    a b c d e f g h i j k l m n o p q r s t

    a b c d e f g h i j k l m n o p q r s t

    a b c d e f g h i j k l m n o p q r s t

    On most LCD displays there is memory for 40 characters on each line. Each space in

    the RAM memory can be thought of as a ‘box’ which is ready to hold a single

    character. Each RAM ‘box’ has a numbered address to describe it. The first line RAM

    ‘boxes’ are at addresses 128 to 191, the second line RAM ‘boxes’ are from 192 to 255.

    16x2 displays have a window that is two lines deep. That means that 16 letters can be

    seen on each line. If a character is to be printed on the second line, it is necessary to

    move the cursor to the start of line 2. Moving the cursor is very simple; simply send

    the RAM address (of the ‘box’ to be moved) as an instruction. Therefore to move the

    cursor to the start of the second line, simply send the instruction ‘192’ to the LCD

    module. To move the cursor to the fifth position on the second line send the

    instruction ‘197’ (=192+5).

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    © copyright 2000

    Note about 16x1 displays...Note about 16x1 displays...Note about 16x1 displays...Note about 16x1 displays...Note about 16x1 displays...Most 16x1 LCDs are in actual fact 8x2 LCDs, but with the ‘second’ line positioned

    directly after the first (instead of underneath it). The reason for this is that it is cheaper

    to make an 8x2 display than a 16x1 display, as only one ‘LCD controller’ integrated

    circuit is required instead of two (look for the black ‘square’ on the rear of the LCD

    module). However, this makes 16x1 displays confusing to use, as, after 8 characters

    have been printed, the cursor seems to disappear in the middle of the display! If this

    type of display is needed, remember that the ‘ninth’ character is actually the first

    character of the second line. This problem does not occur with 16x2 or larger displays,

    and so, for ease of use, it is probably worthwhile spending the extra money to buy a

    16x2 instead of a 16x1.

    Connecting The LCD (OPTION 1)Connecting The LCD (OPTION 1)Connecting The LCD (OPTION 1)Connecting The LCD (OPTION 1)Connecting The LCD (OPTION 1)The serial LCD firmware is used to allow serial control of an alphanumeric LCD. This

    allows microcontrollers (and microcontroller based systems such as the PICAXE or

    Basic Stamp) to visually output user instructions or readings onto a text screen without

    the need for a host computer. This is especially useful when working, for example,

    with analogue sensors, as the analogue reading can easily be displayed on the LCD

    module. All LCD commands are transmitted serially via a single microcontroller pin.

    A sample instruction, using the serout command is as follows:

    to print the text ‘Hello’ the instruction is simply

    serout 7,T2400,(“Hello”)

    LCD

    serial

    LCD

    firmware

    PICsingle

    pin

    5V

    0V

    5V

    0V

    123456789

    181716151413121110

    +5V

    0V

    serialinput

    4 MHz4k7

    reset

    LC

    D F

    IRM

    WA

    RE

    D7

    D6

    D5

    D4

    RS

    E

    14

    13

    12

    11

    4

    6

    Pin 9

    Pin 8

    Pin 7

    Pin 6

    Pin 1

    Pin 2

    10k0V

    Vdd V0 Vss R/W D0 D1 D2 D3

    2 3 1 5 7 8 9 10

    Pin 5

    Pin 18

    Pin 17

    680R

    0V

    conn

    ectio

    ns to

    LC

    D fi

    rmw

    are

    For more information, see the Serial LCD Firmware datasheet at www.rev-ed.co.uk

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    © copyright 2000

    Connecting The LCD (OPTION 2)Connecting The LCD (OPTION 2)Connecting The LCD (OPTION 2)Connecting The LCD (OPTION 2)Connecting The LCD (OPTION 2)

    The LCD has 6 lines that can be connected directly to the PIC microcontroller pins.

    However it is a good design practice to add a low value resistor (e.g. 330R) on the

    lines to protect against static discharges. The 10k potentiometer connected to pin 3 is

    used to adjust the contrast of the display. All unused lines should be tied to ground as

    shown.

    DB4

    DB5

    DB6

    DB7

    SE

    RS

    14

    13

    12

    11

    6

    4

    6 x 330R

    Pin 7

    Pin 6

    Pin 5

    Pin 4

    Pin 3

    Pin 2

    +5V10k

    0V

    Vdd V0 Vss R/W DB0 DB1 DB2 DB3

    2 3 1 5 7 8 9 10

  • 3535353535 Section 3Section 3Section 3Section 3Section 3

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    © copyright 2000

    A Simple LCD ProgramA Simple LCD ProgramA Simple LCD ProgramA Simple LCD ProgramA Simple LCD Program

    The following program will print out the phrase ‘Hello there!’ on two lines of the LCD

    display. It uses three sub-procedures called init , wrins and wrchr . These three sub-

    procedures carry out all the ‘difficult’ software tasks, and are ‘standard’ sub-procedures

    that will not have to be changed. In fact they can be used without understanding how

    they work, but it is necessary to know what they do:

    init ‘initialises’ the LCD so that it is ready to accept instructions

    wrins sends an instruction stored in variable b1 to the LCD module

    wrchr sends a character stored in variable b1 to be ‘printed’ on the LCD screen

    The three sub-procedures are explained further in the following sections.

    EEPROM 0,(“Hellothere!”) ‘ store the text in the EEPROM memory

    gosub init ‘ initialise LCD

    main: let b1 = 1 ‘ set b1 to ‘clear display’ instruction

    gosub wrins ‘ send instruction to LCD

    for b3 = 0 to 4 ‘ setup for...next loop (“Hello” - positions 0 to 4)

    read b3, b1 ‘ read letter from EEPROM into variable b1

    gosub wrchr ‘ send character to LCD

    next b3 ‘ next loop

    let b1 = 192 ‘ set b1 to ‘start of second line’ position

    gosub wrins ‘ send instruction to LCD

    for b3 = 5 to 11 ‘ setup for...next loop (“there!”-positions 5 to 11)

    read b3, b1 ‘ read letter from EEPROM memory into variable b1

    gosub wrchr ‘ send character to LCD

    next b3 ‘ next loop

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    More Advanced LCD ProgramMore Advanced LCD ProgramMore Advanced LCD ProgramMore Advanced LCD ProgramMore Advanced LCD Program

    The following program scrolls the message ‘Hello there everybody!’ across the screen.

    As the text is longer than 16 letters, the message is first stored in the LCD memory, and

    then the display window is repeatedly scrolled to show all the message.

    EEPROM 0,(“Hello there everybody!”) ‘ store the text in the EEPROM memory

    gosub init ‘ initialise LCD

    start: let b1 = 1 ‘ set b1 to ‘clear display’ instruction

    gosub wrins ‘ send instruction to LCD

    for b3 = 0 to 22 ‘ setup a for...next loop

    read b3, b1 ‘ read letter from EEPROM into variable b1

    gosub wrchr ‘ send character to LCD

    next b3 ‘ next loop

    let b1 = 12 ‘ set b1 to ‘hide cursor’ instruction

    gosub wrins ‘ send instruction to LCD

    main: let b1 = 24 ‘ set b1 to ‘scroll display left’ instruction

    gosub wrins ‘ send instruction to LCD

    pause 250 ‘ pause for 0.25s

    goto main ‘ loop

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    © copyright 2000

    Standard LCD Sub-ProceduresStandard LCD Sub-ProceduresStandard LCD Sub-ProceduresStandard LCD Sub-ProceduresStandard LCD Sub-Procedures

    Before the sub-procedures are studied, it is important to understand how the LCD

    module operates. It has two modes of operation, which are called ‘character’ mode and

    ‘instruction’ mode. The RS pin (pin 2) controls the mode - when high the LCD is in

    character mode, when low the LCD is in instruction mode.

    The character or instruction is sent as a 4 bit binary number down the data lines (pins

    7-4). Every time the Enable pin (pin 3) is ‘pulsed’ the LCD reads the data lines and

    prints the character (or carries out the instruction) which is given by the number on

    the data lines.

    This is not quite the whole story, as each character or instruction is actually made up of

    an 8 bit number, which contains a table of all the character and instruction codes. As

    there are only four data lines, this 8 bit number is split into two ‘halves’ which are sent

    one after the other. The two halves are called the ‘high nibble’ and the ‘low nibble’.

    This means that two nibbles are transmitted down the data lines for each character.

    1011 0101 = 10110101

    high nibble + low nibble = byte

    The three ‘standard’ sub-procedures described below perform all of the ‘complicated’

    software tasks when using the LCD display. Each sub-procedure is called from the

    main program to perform a certain task. These tasks are:

    init initialise the display and sets the module to two line operation

    wrchr ‘prints’ one ‘character’ onto the LCD screen

    wrins writes one ‘command’ to the LCD module.

    (This is actually just the wrchr sub-procedure with the addition of one line

    that sets the RS line into ‘instruction’ mode at the start of the sub-

    procedure).

    DB4

    DB5

    DB6

    DB7

    SE

    RS

    14

    13

    12

    11

    6

    4

    6 x 330R

    Pin 7

    Pin 6

    Pin 5

    Pin 4

    Pin 3

    Pin 2

    +5V10k

    0V

    Vdd V0 Vss R/W DB0 DB1 DB2 DB3

    2 3 1 5 7 8 9 10

  • 3838383838 Section 3Section 3Section 3Section 3Section 3

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    © copyright 2000

    init: let pins = 0 ‘ Clear all output lines

    let dirs = 252 ‘ Set pins 2-7 as output lines.

    pause 200 ‘ Wait 200 ms for LCD to reset.

    let pins = 48 ‘ Set to 8-bit operation.

    pulsout 3,1 ‘ Send data by pulsing ‘enable’

    pause 10 ‘ Wait 10 ms

    pulsout 3,1 ‘ Send data again

    pulsout 3,1 ‘ Send data again

    let pins = 32 ‘ Set to 4-bit operation.

    pulsout 3,1 ‘ Send data.

    pulsout 3,1 ‘ Send data again.

    let pins = 128 ‘ Set to two line operation

    pulsout 3,1 ‘ Send data.

    let b1 = 14 ‘ Screen on, cursor on instruction

    gosub wrins ‘ Write instruction to LCD

    return

    wrins: low 2 ‘ Change to instruction mode

    wrchr: let b2 = b1 & 240 ‘ Mask the high nibble of b1 into b2.

    let pins = pins & 7 ‘ Clear the data lines.

    let pins = pins | b2 ‘ Put the contents of b2 onto data lines.

    pulsout 3,1 ‘ Pulse the enable pin to send data.

    let b2 = b1 * 16 ‘ Put low nibble of b1 into b2.

    let b2 = b2 & 240 ‘ Mask the high nibble of b2

    let pins = pins & 7 ‘ Clear the data lines.

    let pins = pins | b2 ‘ Put the contents of b2 onto data lines.

    pulsout 3,1 ‘ Pulse enable pin to send data.

    high 2 ‘ Back to character mode

    return

    Note that init uses a let dirs = commands that will affect all 8 pins, not just the 6used by the LCD display. The let pins = commands used by wrins/wrchr will notalter the state of unused pins 0 and 1. Do not use variable b1 or b2 (or w0 or w1) forany other function within a program.

    NB. The | character is ‘SHIFT + \’ (next to Z) on a UK keyboard.

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    Using the LCD Instruction setUsing the LCD Instruction setUsing the LCD Instruction setUsing the LCD Instruction setUsing the LCD Instruction set

    The codes for the LCD instructions are given below. Each code can be sent to the LCD

    module by using the wrins sub-procedure. These instructions can be used to make the

    LCD messages more interesting - for instance by flashing the screen or creating

    ‘moving’ messages which scroll across the screen.

    CodeCodeCodeCodeCode InstructionInstructionInstructionInstructionInstruction

    1 Clear display and move to the start of the first line

    2 Move the cursor and display ‘window’ to the start of the first line

    4 Set ‘right to left printing’ mode

    5 Set ‘scroll printing to the left’ mode

    6 Set ‘left to right printing’ mode

    7 Set ‘scroll printing to the right’ mode

    10 Turn visual LCD screen off

    12 Hide cursor

    13 Make cursor flash

    14 Turn visual LCD screen (and cursor) on

    16 Move cursor left one position

    20 Move cursor right one position

    24 Scroll display ‘window’ left one position

    28 Scroll display ‘window’ right one position

    128 Move cursor to the start of the first line

    192 Move cursor to the start of the second line

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    Examples:

    Clear the display

    clear: let b1 = 1 ‘ Set b1 to clear instruction

    call wrins ‘ Send it to LCD

    Move cursor to the second line

    clear: let b1 = 192 ‘ Set b1 to start of second line

    call wrins ‘ Send it to LCD

    Flash a message 10 times

    flash: for b3 = 1 to 10 ‘ Start a for...next loop using

    ‘ variable b3 Don’t use b1!!

    let b1 = 10 ‘ Set b1 to ‘turn visual display

    ‘ off’ instruction

    gosub wrins ‘ Send instruction to LCD

    pause 200 ‘ Pause for 0.2 second

    let b1 = 14 ‘ Set b1 to ‘turn visual display

    ‘ back on’ instruction

    gosub wrins ‘ Send instruction to LCD

    pause 200 ‘ Pause for 0.2 second

    next b3 ‘ End of for...next loop

    Scroll a long message (30 characters long)

    scroll: for b3 = 1 to 30 ‘ Start a for...next loop using

    ‘ variable b3 Don’t use b1!!

    let b1 = 28 ‘ Set b1 to ‘scroll display

    ‘ window right’ instruction

    gosub wrins ‘ Send instruction to LCD

    pause 200 ‘ Pause for 0.2 second

    next b3 ‘ End of for...next loop

    let b1 = 1 ‘ Set b1 to ‘move scroll window

    ‘ back to start’ instruction

    gosub wrins ‘ Send instruction to LCD

    pause 200 ‘ Pause for 0.2 second

    goto scroll ‘ Loop

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    Advanced Interfacing 2 - Serial Interfacing to a Computer.Advanced Interfacing 2 - Serial Interfacing to a Computer.Advanced Interfacing 2 - Serial Interfacing to a Computer.Advanced Interfacing 2 - Serial Interfacing to a Computer.Advanced Interfacing 2 - Serial Interfacing to a Computer.

    Most computers can ‘talk’ to other devices by serial communication. Serial

    communication uses a common ‘protocol’ (or code) where characters are converted

    into numbers and then transmitted via cables. A computer mouse normally

    ‘communicates’ serially with a computer, and computer modems work by turning

    these numbers into sounds to travel down telephone lines.

    As all computers use the same ASCII code for transmitting and receiving characters it is

    relatively easy to program the PIC microcontroller to ‘talk’ to any type of computer. All

    that is needed is a suitable cable and some very simple electronic circuits.

    Connecting to the ComputerConnecting to the ComputerConnecting to the ComputerConnecting to the ComputerConnecting to the Computer

    The system we will use requires just three wires between the computer and the

    microcontroller. The ground wire provides a common reference, the RX wire sends

    signals from the computer to the PIC microcontroller, and the TX wire sends signals

    from the PIC microcontroller to the computer.

    The best way to make a serial cable is to buy a serial ‘extension’ cable and cut it in

    half. This will give two cables with a suitable connector at each end. The diagrams

    below show the various wiring connections required.

    Computer Communication SoftwareComputer Communication SoftwareComputer Communication SoftwareComputer Communication SoftwareComputer Communication Software

    To use this system a communication software package is required for the PC. The

    examples below use the Terminal option within the Programming Editor software, but

    any communications package can be used.

    There are various different protocols that can be used for serial communication, and it

    is important that both the computer and the microcontroller use the same setting. The

    2400,N,8,1 protocol is used here, which means baud speed 2400, no parity, 8 data bits

    and one stop bit. This baud speed is quite slow by modern standards, but is quite

    sufficient for the majority of project work tasks. All ‘handshaking’ (hardware or

    software) must also be disabled.

    PC/RISC PC PC (25 way) Mac

    5 3 2 7 3 2 5 4 3

    RX = 3TX = 2GND = 5

    RX = 3TX = 2GND = 7

    RX = 3TX = 5GND = 4

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    © copyright 2000

    PIC Microcontroller Interfacing CircuitPIC Microcontroller Interfacing CircuitPIC Microcontroller Interfacing CircuitPIC Microcontroller Interfacing CircuitPIC Microcontroller Interfacing Circuit

    The system described here requires just three wires between the computer and the PIC

    Microcontroller. Strictly speaking RS232 serial voltages should be at ±15V, but the

    standard 5V from the on-board 5V regulator will be used here. This is not the industry

    standard, but works perfectly OK with the majority of computers. This is the circuit

    that will be used use for serial communication.

    To provide true RS232 voltages another integrated circuit is required. The most

    common IC used is the MAX232, which has on-board voltage boosters to create the

    required voltage swing. If this setting is used it is necessary to change the N2400

    (negative) in all the serial software commands to T2400 (true positive).

    0V

    10k

    22kPin 0

    180RPin 1

    0V

    RX

    TXTocomputer To PIC

    MA

    X 232

    1

    2

    3

    4

    5

    6

    7

    8

    16

    15

    14

    13

    12

    11

    10

    9

    +

    +

    +

    +

    10uF

    10uF

    10uF

    10uF

    5V

    0V

    TX

    RX

    Pin 0

    Pin 1

    NC

    NC

    NC

    NC

    0V

    Computer

    PIC

    NB. Note polarity - capacitors connected topins 2 and 6 are connected upside down.

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    Transmitting Characters to the Computer ScreenTransmitting Characters to the Computer ScreenTransmitting Characters to the Computer ScreenTransmitting Characters to the Computer ScreenTransmitting Characters to the Computer Screen

    The following program will transmit the word ‘Hello’ to the computer screen over and

    over again. If the cable is connected and the communication software is operating

    correctly, the word will appear every second.

    main: serout 1,N2400,(“Hello”) ‘ Send the word ‘Hello’

    serout 1,N2400,(10,13) ‘ Send the ‘new line’ instructions.

    pause 1000 ‘ Wait one second

    goto main ‘ Loop back to the start

    Notice that “text” must be enclosed within speech marks. This tells the microcontroller

    to convert the text into a string of ASCII codes. Individual ASCII codes can be

    transmitted by just giving their numbers. Therefore the two commands below achieve

    the same task:

    serout 1,N2400,(“Hello”)

    serout 1,N2400,(72,101,108,108,111)

    Receiving Keyboard Input from the ComputerReceiving Keyboard Input from the ComputerReceiving Keyboard Input from the ComputerReceiving Keyboard Input from the ComputerReceiving Keyboard Input from the ComputerIt can be useful to be able to use a keyboard for people to ‘answer’ questions. This is

    achieved by using the serin command as shown below.

    main: serout 1,N2400,(10,13) ‘ Start a new line

    serout 1,N2400,(“Press a key- “) ‘ Send a message

    serin 0,N2400,b1 ‘ Receive a character into variable b1

    serout 1,N2400,(b1) ‘ Transmit character back to the screen

    if b1=”a” then hot ‘ Is character ‘a’? If yes goto hot

    goto main ‘ No, so loop back to start

    hot: serout 1,N2400, (10,13,”A is the Hot Key!”)

    ‘ Send message

    goto main ‘ Loop back to start

    If this program is run and then a key is pressed on the keyboard, the character will

    appear on the screen. This is the microcontroller (not the computer) working. The

    keyboard press has been received from the keyboard and then transmitted back to the

    screen!

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    Characters or numbers?Characters or numbers?Characters or numbers?Characters or numbers?Characters or numbers?

    Consider this command: serout 1,N2400,(65)

    This will send the ASCII character ‘A’ to the screen.

    Now consider this command: serout 1,N2400,(b1)

    This will send the character stored in variable b1 to the screen, and so if b1=65, the

    character ‘A’ will be sent to the screen.

    However, variables are often used to store the answers to mathematical sums, and so it

    may be necessary to send the number ‘65’ to the screen rather than the letter ‘A’. To do

    this, the microcontroller must be told that a number is to be sent rather than a

    character. This is achieved by adding a hash (#): serout 1,N2400,(#b1)

    This will send the number ‘65’ (actually the two characters ‘6’ and ‘5’) to the screen

    rather than the character ‘A’.

    This is a summary of the serial commands used. Remember that the pin number may

    have to be changed, and also to the N2400 section to P2400 if the MAX232 interfacing

    circuit is used.

    serout 1,N2400,(“Hello”)- Sends a message to the screen.

    serout 1,N2400,(10) - Sends a direct ASCII instruction to the screen.

    serout 1,N2400,(b1) - Sends an ASCII character stored in variable to

    the screen.

    serout 1,N2400,(#b1) - Sends a number stored in a variable to the

    screen.

    serin 0,N2400,b1 - Receives an ASCII character from a keypress on

    the keyboard and stores it as the ASCII value in

    a variable (b1)

    serin 0,N2400,#b1 - Receives a real number from the number keys

    on the keyboard and stores it in a variable (b1)